Where railway tracks or other priority traffic cross a highway or pedestrian right of way it is often desired to afford protection by a mechanism which alarms and displays the impending crossing of the priority vehicle, such as a train. Grade crossing mechanisms often include a gate mechanism in which an arm is lowered prior to the train entering the highway crossing to signal motorists to stop. Similar crossing arms can also be used to signal at pedestrian crossings. Because of the nature of these mechanisms it is often desirable that the crossing arm be held in its upward position, and should power or control be lost, gravity will bring the arm down to a horizontal or blocking position. To balance the weight of the extended arm, counter weights are often used. In applications where the arms are extensive, the counter balance can become quite heavy. To raise the arms the highway crossing mechanism utilizes a motor, usually acting through a gear train to drive the crossing arm to a vertical or up position. To maintain high accuracy, avoid backlash in the gear train, and maintain high reliability, the position of the crossing arm is sensed by cam switches which act upon cams attached directly to the output shaft which drives the crossing arm.
One of the principle functions of the circuit controller, and the cam operated switches in particular, is to control the "on" and "off" operation of the current applied to the drive motor during the drive "up" mode. The point at which this set of contacts must turn the motor off is also the point at which the motor is quite often under its heaviest load. Because the motor, which is usually a D.C. motor, is operating at its maximum current at this time, the cam operated switch is interrupting the maximum motor current flowing through the contacts. Such operation can result in severe burning or erosion of contacts. In applications where D.C. motors are used, the current interruption can be difficult due to the inductive nature of the motor armature. For these reasons it would be desirable to have a contact which opens very fast. However, because the output shaft is moving at a very slow speed, often less than two revolutions per minute, it is inherently difficult to open the contacts quickly on a cam operating directly from the slow speed output shaft.
The cam operated switches also sense the position of the arm to institute the stop positions for both the horizontal and the vertical crossing gate arm positions. Therefore it is desirable to have the cam accurately sense such position when turning the motor current off, such that a motor mounted brake can be set on the motor shaft to hold the crossing arm in the full-up position. Highway grade crossing arms can be up to forty-five feet long, and are often made of hollow aluminum or fiberglass. These crossing arm mechanisms are subject to vibration or oscillation when the arm mechanism comes to a stop. This vibration or oscillation in the crossing arm can cause the cam switch to be momentarily driven backwards over the shut-off point, and thereby reenergize the drive-up motor. Such oscillations around the turn-off points are highly undesirable as they can cause mis-positioning of the arm, or damage to portions of the mechanism. Therefore it is desirable to have a switch which senses the position of the arm to turn off the drive motor very accurately, but not be over sensitive to oscillations or slight roll backs in the drive mechanism after the power has initially been interrupted.
Cam operated limit switches are well-known and may include devices which have a contact mounted directly on a cam such that the lobe engages the switch contact to move it from a first to a second position. The speed with which the contact changes position can be controlled somewhat by the slope between the lobe area and the inner radius of the cam. Such profiling is limited to the mechanical ability of the contact engaging the slope area and the rotational speed of the cam. It is also known to use a contact which rides on a cam wherein the contact has a shoe portion which engages the lobe and cam. Respective profiling between the slope areas on the lobe and the shoe can also be used to control the opening profile of the switch. If it is desired to open the contact very quickly, a steep profile on the slope area between the cam lobe and the inner radius of the cam is often desirable. However, because the cam will usually operate in a bi-directional rotational mode, it will usually be necessary that the switch contact also must ride upon or follow the slope area during reverse operation. If the steepness of the slope becomes too great, the contact may not reliably ride upward during reverse rotation. In some prior art cam operated switches it has been known to use a roller which is free to rotate about a fixed axis on the switch member. The roller then engages the cam and the lobe and can roll both up and down the slope areas between the lobe and the inner radius of the cam. The use of the roller greatly reduces the friction between the switch contact and the cam member. However, restrictions still apply as to the steepness of the slope and the amount of control that can be provided for quick opening on slow moving shafts.